Immunological memory is a defining characteristic of Immunity, one that has allowed us to develop vaccines, some with the power to provide life-long immunity to pathogenic infectious diseases. While neutralizing antibodies have often been the focus of vaccination strategies, cell mediated protection, mainly in the form of cytotoxic CD8 T cells, also plays an important role in fighting many infections. CD8 T cells are crucial for killing infected cells, preventing the spread of pathogens, and even killing some cancer cells. Long-term CD8 T cell protection comes from memory CD8 T cells, which have a high proliferative capacity, can self renew, and respond almost immediately upon a secondary infection. However, the memory population is not uniform and at least two subsets can be identified, effector memory cell (TEM) and central memory cells (TCM). TCM have and increased capacity to self renew and proliferate, while TEM seem to have enhanced effector functions. Yet there is still a poor understanding of how these subsets develop and whether they represent different lineages or can interconvert in response to environmental signals. Given that memory cells can survive for decades it's important to understand the functional contribution of these subsets and how they are maintained. Previous work from our lab has shown that inflammatory signals can induce the transcription factor T-bet which modulates the potential of an effector CD8 T cell to become a memory cell by prompting the formation of short lived effector cells (SLECs) with reduced potential to form memory. We propose to investigate whether T-bet also plays a role in influencing the differention state of memory subsets through inflammatory signals in their environment. We will also further investigate how T-bet induces effector stage cells to terminally diferentiate. Understanding how CD8 Tcells acquire, or lose, memory characteristics is crucial for the development of more efficacious vaccines to protect from infectious diseases and potentially cancer. We plan on addressing these questions through the following aims: Specific Aim 1;To determine whether the differentiation state of SLECs can be modulated by reducing T- bet, in the absence of antigen. Specific Aim 2: To determine if TEM and TCM memory CD8 T cell subsets interconvert and if T-bet is required for this process. Specific Aim 3: To determine if the transcription factor ZEB, a candidate target of T-bet, co-regulates the terminal differentiation of CD8 T cells.